The classic neurological approach in attributing functional processes to various regions of the human cerebral cortex involved initially describing sensor motor, cognitive and behavioral deficits and then awaiting patiently, sometimes years, for the final pathology reports describing the actual anatomical lesions. This inability to localize cortical lesions in living humans, independently of signs and symptoms, has slowed the refinement of maps that correlate cortical function to cortical structure. In recent years, the developments of Computerized Tomography (CT) and Magnetic Resonance Imaging (MRI) have greatly helped to overcome the aforementioned problems in localizing the anatomic lesion.
While the visualization of grey and white matter in the brain and the localization of neocortical lesions by scanning techniques are sufficient for making neurologic diagnoses, other obstacles remain in correlating cortical function to the brain structure, e.g., the complex convolutional geometry of the human cerebral cortex and the extreme variability of neocortical folding patterns between humans.
The variations in the sulcal and gyral patterns of the human neo-cortex are as diverse as fingerprints in the population and greatly complicate detailed correlation between specific cortical zones and functional processes. Although the gyri remain the best anatomical landmarks to the functional zones of the living cortex, it is unclear to what extent a lesion in a given gyrus in one individual is functionally comparable to a similarly placed lesion in the same gyrus in another individual.
Current brain maps which typically depict functional zones over lateral views of the cortex are inadequate. Though useful for instructional purposes, such schematics of the cortex make no attempt to open the sulci, or show which functions are therein processed and seem not to appreciate that most neocortical tissue is buried within the sulci.
Cortical mapping methods fall into one of two broad families, depending on whether or not they preserve or distort the natural geometry of the cerebral cortex. Computer programs that rotate in space, three-dimensional images of the brain are an example of techniques that strive to replicate, as absolutely as possible, the natural geometry of the cortex. While these methods minimize distortion, they do not simplify the cortical geometry.
Flat maps of the cortex comprise a second category in which distortion is introduced, but neocortical geometry is simplified. Using physical methods, investigators have determined the surface area of human cortex by staining, enlarging, and tracing histologic sections, straightening and aligning each, tracing along one end of given fissures, and measuring areas contained therein. Others have assessed the total and regional surface areas of a human brain by measuring slice perimeters, multiplying perimeter lengths by slice thicknesses and interslice distances, summing the products and correcting the sum for shrinkage. An advance in this field was achieved in 1980 by Van Essen and Maunsell and is described in "Two-Dimensional Maps of the Cerebral Cortex", the Journal of Comparative Neurology, Vol. 191, pp. 255-281 (1980). Van Essen et al. describe a method for producing a single cortical map which illustrated whole monkey and cat hemispheres. Their method employs deriving cortical ribbons of successive histologic sections which are partially unfolded around one another. Cortical surface areas are then measured planimetrically. The Van Essen et al. technique involves a labor-intensive manipulation of the contour lines of various cortical layers onto a composite two-dimensional representation, with each contour line being held constant in length but allowed to change its shape in order to be aligned properly with respect to contour lines from nearby sections. This procedure introduces area distortions to the map in an attempt to reduce angular distortions.
Other investigators have employed a "straight-line two-dimensional cortical flat map" technique (SL2D) and applied it to various comparative anatomical studies to produce two-dimensional flat maps of unfolded cortex matter. Putnam in "Studies of the Central Visual System" Archives of Neurology and Psychiatry, Vol. 16 No. 6, December 1926, pp. 683-707 used the SL2D method to examine surface areas of the human calcarine fissure. These slices were histologically stained, projected and then traced. A map was constructed and estimations of surface cortical area were obtained by placing the map over a ruled grid and then estimating the areas. This straight-line method has been applied over the years by various investigators (i.e. see Jouandet, Lachat and Gary "Topographic Distribution of Calosal Neurons and Terminals in the Cerebral Cortex of the Cat", Anatomy and Embryology, Vol. 173, 1986, pp. 323-342); however none of the methods have succeeded in reducing angular distortion or enabling adjustment of the maps to provide substantially distortion free areas of interest.
Whenever a 3-D surface is represented on a 2-D map, three types of distortion may be introduced, areal, angular and topologic, to various degrees, depending on how the 2-D map is constructed.
Accordingly, it is an object of this invention to provide a method for mapping a three-dimensional surface onto a two-dimensional surface wherein area distortions are minimal.
It is a further object of this invention to provide a method for mapping three-dimensional surfaces onto a two-dimensional map wherein overall angular distortions are minimized.
It is still another object of this invention to provide an improved method for mapping with lessened distortion, the human cortex onto a two-dimensional planar map.
It is yet another object of this invention to provide an improved cortical mapping method which enables rapid and automatic map production.